Wireless power transfer systems have been receiving increased attention in response to expanding popularity and availability of battery-powered handheld electronic devices. Some wireless power transfer systems use near-field electromagnetic coupling (e.g., mutual inductance) to charge electronic devices by transferring power from a transmitter winding (“primary winding”) located external to a device to a receiver winding (“secondary winding”) within the device. Wireless connections can provide a number of advantages over conventional hardwired connections, including a high degree of electrical isolation between the transmitter and receiver circuits. Nonetheless, relatively reduced levels of power transfer efficiency have often limited inductive power transfer systems to niche applications. One effort to improve power transfer efficiency is disclosed in U.S. Pat. No. 7,411,479 to Baarman et al., entitled “Inductive Coil Assembly.”
As will be understood by those skilled in the art, because a resonant tank circuit within the power transfer system may operate at relatively high frequency, the skin effects of winding conductors should be minimized; otherwise, eddy current losses may be unacceptably high and power transfer efficiency may be unacceptably low. Various techniques have been developed to reduce eddy current losses in high frequency applications. These techniques can include using Litz wire, which consists of thin wire strands that are individually insulated and twisted or woven together, and reduced-thickness copper foil. In addition to increasing power transfer efficiency, the configuration and layout of the primary and secondary windings should also be sufficient to comply with the International commission on Non-Ionizing Radiation Protection Guidelines (ICNIRP) in order to limit human exposure to time-varying EMFs.